The total number of Apple devices sold will equal the number of Windows PCs by some point this year, predicts Asymco’s Horace Dediu in an interesting piece of analysis.

The dark shaded area compares all Apple hardware – Mac plus iPad plus iPhone – with Windows PCs, and shows that by last year there were only 1.18 more Windows PCs than Apple devices. It’s of course a somewhat artificial comparison, as Dediu is including iPhones while excluding Windows Phones and tablets, but given the very limited success of Windows mobile devices to date, correcting that wouldn’t change the patterns too much.

What’s particularly interesting here, as John Gruber notes, is that the dramatic reversal kicked in well before the launch of the iPhone. Dediu and Gruber between them put forward a number of theories for this, and of these I think three are likely key …